In the industry, hotmelt processes employing solvent-free coating technology are of growing importance for the preparation of pressure-sensitive adhesive (PSA) compositions. Generally speaking, environmental regulations and rising costs are pushing forward this development process. As well as SIS (styrene-isoprene-styrene copolymer) systems, acrylic polymers are increasingly being applied from the melt as a polymer film to backing materials. Moreover, for specialty applications, there is a need for PSA tapes having very low outgassing characteristics. Such characteristics can be ensured only by means of hotmelt processes, since conventional coatings applied from a solution always contain small residual solvent fractions.
The hotmelt process commonly starts with a solution polymerization followed by a concentration process in which the solvent is removed. A variety of techniques are known for doing this.
The switch to hotmelt processes imposes particular requirements in respect of a downstream crosslinking procedure. Thus, for example, thermal crosslinking techniques using metal chelates or polyfunctional isocyanates, which are widespread with the solvent systems, can no longer be employed. At the present time, therefore, the crosslinking of polyacrylate PSA compositions is forced by exposure to ultraviolet light (UV) or electron beams (EB), the latter being known as electron beam curing (EBC).
For the production of acrylic PSA tapes with low outgassing characteristics, UV crosslinking is preferential. EB crosslinking is a random process which as well as actually crosslinking the polymer chains produces fragments which remain in the PSA tape and may subsequently outgas when the bond is exposed to temperature.
Generally speaking, UV crosslinking is a process which does not involve very complex apparatus, needing only a simple coating unit with a few low-pressure Hg lamps. UV crosslinking operates very effectively for polyacrylate compositions with film thickness up to 100 g/m2. The EBC technology, in contrast, is significantly more complicated.
For industrial applications, especially in the electronics sector, temperature-stable, shearing-resistant acrylic PSA tapes with very low outgassing are required. Residues of solvent and residual monomer may evaporate out during application at elevated temperatures, with the consequence that the atmosphere may contain solvent. This can give rise to a variety of problems. One is that certain monomers possess good conductivity, so that the risk of short circuits increases. Another is that the solvents and/or monomers may bring about the destruction of plastic parts and insulators. In this sector, therefore, attempts are made to use very high-purity acrylic PSA tapes which possess extremely low outgassing tendencies.
The U.S. Pat. No. 5,681,654 has already described a pressure-sensitive adhesive composition having very low outgassing (fogging) characteristics. This PSA composition, however, was created specifically for applications in automaking, where relatively high levels of outgassing are tolerated. Moreover, this patent describes not acrylic PSA compositions but rather rubber adhesive compositions. These elastomers have a number of fundamental disadvantages relative to polyacrylates, such as, for example, the lower thermal shearing resistance and the tendency to age by oxidation of the double bonds present. Since in the electronics sector it is often possible for ozone to form (in copiers, for example), the rubber adhesive compositions specifically are completely unsuitable for such a use.
The U.S. Pat. No. 5,761,184 described vibration-damping materials which likewise possess little tendency to outgas. The limit of tolerable outgassing, however, was not defined. Furthermore, for application of the damping material, a double-sided acrylic PSA tape was used. Here again, there was no more detailed characterization of the PSA tape nor investigation of the outgassing characteristics of the assembly.
DE 198 07 752 A1 introduces a nonfogging adhesive tape, fogging referring to the condensation of evaporated volatile components from the automotive interior trim on the windows. This self-adhesive tape comprises a nonfogging backing on at least one side of which a nonfogging PSA composition has been applied. The adhesive composition used is preferably one based on an acrylic hotmelt. For the residual solvent content, a preferred limit of at most 1% by weight is stated.
In addition to the abovementioned removal of the solvent from the polymerization process, therefore, for specific application of the polyacrylate composition produced it is necessary to conduct a postpurification in which not only remaining solvent residues but also residual monomer residues are removed. This postpurification process may be conducted in conjunction with the actual concentration process.
One proposal for a postpurification is given by DE 43 40 136 A1. That document describes a process in which the removal of the solvent takes place under reduced pressure and, toward the end of the distillation, entrainers such as steam, nitrogen, argon, and CO2 are supplied at temperatures of more than 100° C. For this process, however, it is necessary to pump the system in circulation in order to optimize polymer mixing. A disadvantage of this process is that the pump circulation procedure places limits on the viscosity of the solution to be concentrated, thereby making it impossible to prepare end products of relatively high viscosity.
BASF has developed a relatively low molecular mass acrylic hotmelt possessing a UV crosslinking mechanism [U.S. Pat. No. 5,073,611]. The copolymerized benzophenone photoinitiator is a type II photoinitiator, which does not release fragments under UV exposure and thus during crosslinking. These pressure-sensitive adhesive compositions possess little outgassing tendency. Nevertheless, owing to the concentration process, the residual monomer fraction is still too high and is situated above a preferred limit of 10 μg/g adhesive composition.
DE 43 13 008 A1 and EP 621 326 B1 describe the use of an extruder to concentrate a self-adhesive composition based on acrylic hotmelt. In this case an extruder is used to concentrate or devolatilize a self-adhesive composition of this kind having a K value of at least 60, to give a system which can be processed as a hotmelt for a plaster or for an industrial adhesive tape. In one preferred embodiment of the invention, furthermore, the residual solvent content is said to fall below 1% by weight.
It is an object of the invention to provide a pressure-sensitive adhesive composition, particularly for application in the electrical and electronics sectors, which does not have the disadvantages of the prior art. In particular, for pressure-sensitive adhesive compositions used in such applications, the intention is to reduce the presence of monomers and fragments which may lead to short circuits and the like.
Surprisingly and unforeseeably, pressure-sensitive adhesive compositions with significantly reduced outgassing characteristics have been prepared for the first time; consequently, these compositions are outstandingly suitable for use in this field of application.